A scanning electron microscope is configured to accelerate electrons emitted from an electron source of thermal type or field emission type to form a thin electron beam (primary electron beam) using an electrostatic lens or a magnetic lens, subject the primary electron beam to two-dimensional scanning on a sample surface to be observed, detect a secondary signal, for example, secondary electron or backscattered electron secondarily generated from the sample resulting from radiation of the primary electron beam, and set the detected signal intensity to a brightness modulation input of a cathode-ray tube which is scanned simultaneously with scanning of the primary electron beam so as to provide a two-dimensional scan image. The use of the scanning electron microscope for the process of fabricating semiconductor devices or inspection after completion (for example, critical dimension measurement or electric performance inspection) has demanded high resolution equal to 10 nm or lower at a low accelerating voltage of 1 kv or lower, which allows observation by reducing charging of an insulator.
The method for short focusing by bringing a principal plane of an objective lens close to the focal point has been generally conducted as one of solutions. The method generally called semi-in-lens type has a magnetic pole structure in which the sample is positioned below upper and lower pole pieces of the objective lens so as to positively immerse the sample in the magnetic field. The magnetic pole structure of semi-in-lens type allows space below the objective lens so that a larger sample is handled compared with the in-lens type in which the sample is disposed between the upper and the lower pole pieces. The structure has been widely distributed for the use in the inspection apparatus of the semiconductor wafer.
In the semi-in-lens type magnetic pole structure, high magnetic field is likely to act on the sample surface, which significantly influences a secondary electron trajectory of the secondary electron and backscattered electron generated when irradiating the sample with the electron beam. There are various types of objects to be observed using the scanning electron microscope. Especially when observing the insulator sample, the electron beam radiation generates the secondary electron and the backscattered electron to charge the sample surface, resulting in various types of image degradation. Recently, use of the scanning electron microscope has been increasingly required to observe the sample, for example, reticle which is likely to be charged by the electron beam radiation. Therefore, there is a pressing need to take appropriate measure. Especially, charging makes the potential on the sample surface non-uniform around the electron beam radiation point to generate potential gradient. This may deflect the electron beam trajectory to cause the beam drift phenomenon and an accompanying image blur.
The scanning electron microscope disclosed in Patent Literature 1 is configured to set an incident energy of the electron beam so that a secondary electron emission yield becomes 1 or higher to bring the sample surface into a positively charged state, and control the surface potential distribution in accordance with configurations of the flat-plate electrode and a sample holder for charging control, which are disposed between the sample surface and the objective lens. This makes it possible to observe the reticle and the like. The aforementioned art is effective for the reticle observation, but is difficult to completely remove the influence of charging. Especially for the sample which has the charging state largely changed depending on the incident beam energy, for example, the reticle having the surface coated with resist, stable measurement precision cannot be established. Further reduction of charging is required in order to improve the measurement precision.
The scanning electron microscope disclosed in Patent Literature 2 is configured to provide a magnetic disc independent from the objective lens around a pole piece gap (upper pole piece and lower pole piece) of the objective lens to improve the magnetic field generation efficiency of the objective lens. This may reduce power consumption of the exciting coil as well as the stray magnetic field to the sample surface, which allows observation of the insulator sample with high resolution and high precision by reducing charging. However, the configuration disclosed in Patent Literature 2 significantly degrades the resolution, and does not disclose the specific configuration of the magnetic disc efficient for the charging reduction.
Patent Literature 3 discloses the configuration having the magnetic body disposed below the objective lens. According to Patent Literature 3, the deflector is disposed below the objective lens to generate the magnetic field for beam deflection in order to establish a large incident angle on the sample surface without causing the large deflection of the charged particle beam. The lateral magnetic field is generated using the exciting coil. A deflector pole piece (magnetic plate) is provided to concentrate the generated magnetic field around the optical axis efficiently. However, since the art disclosed in Patent Literature 3 is intended to deflect the beam, the deflector pole piece has four cut portions that constitute four poles, each of which is provided with the exciting coil. The electrodes just above the sample are not rotationally symmetric completely but divided into segments. The electron beam receives the different force in each direction, thus degrading the resolution. As the configuration of the pole piece is not rotationally symmetric, the charging is generated non-symmetrically, and accordingly, the beam drift is emphasized.
The above-described related arts are effective for charging of the insulator sample. However, such effect is limited, and consideration with respect to resolution degradation is insufficient.